The current release does not include cues, but as you’ve evidently seen, the next update will.
It’ll also give you the option to scale (change the size of) pretty much anything.
With that in mine, let me speculate two options:
Cheat! Add the cues as normal notes in the Clarinet part, and add any required key signatures and instrument names manually. Then reduce the size of these elements to make them look like a cue.
Set up a project that contains two players, one holding a Clarinet and the other holding whatever unusual combination of instruments you need to cue into the Clarinet part. Type your cued music into the correct places in the score, and then cue them into the Clarinet part layout.
This really is speculation as I don’t have the next development build of Dorico (no-one does!). I suspect method 1 will be quicker if it’s a short part with not many cues, but method 2 will likely be quicker if it’s a long part, littered with cues.